One of the most common sentence structures in Chapter 6 is sentences ending in 〜のだ、〜んだ、or just 〜の. This structure is used for explanations and sharing information, so it's very conversational. "A Dictionary of Basic Japanese Grammar" explains that the 〜の is used primarily by women. Thinking about it, I suddenly hear the housewives in Ozu's おはよう talking. 「どうしたの？」

Mrs. Weasley

Mrs. Weasley uses this form, when she first meets Harry. It makes her sound kind and motherly.

• 「ロンもそうなのよ。」p. 140
• "Ron's new (a new student), too." p. 70
• 「心配しなくていいのよ。」p. 140
• "Not to worry." p. 70

Hermione

Hermione used it five times in her first meeting with Ron and Harry. When Hermione uses this form, it stresses her bossy, Miss Know-it-All tone of voice; she's treating them like children.

• 「ネビルのが　いなくなったの。」p 157
• "Neville's lost one (a toad)." p. 79
• 「魔法を　かける　の？」p. 157 (かける to start doing something)
• "Oh, are you doing magic? Let's see it then." p. 79
• 「間違ってないの？」 p. 158
• "Are you sure that's a real spell?" p. 79
• 「私の家族に　魔法族は　誰も　いないの。」p. 158
• "Nobody in my family's magic at all..."
• 「参考書を　二、三冊　読んだの。」p.158
• "I got a few extra books for background reading..." p. 79
• 「知らなかったの。」p. 159
• "Goodness, didn't you know?" p. 79

Professor McGonagall

In Chapter 1, McGonagall and Dumbledore have a long conversation in which a lot of explanations are asked for and given. However, she uses this form only once. Her tone of voice is curt and business-like. For an excellent explanation of how Dumbledore speaks in Japanese, see cjvlang site. This site gave me the idea to try to read Harry Potter in Japanese in the first place.

• 「どうして　私だと　おわかりに　なりましたの？」p. 18
• "How did you know it was me?" p. 13

Maybe I'm missing something here. Normally the verb should be informal: なったの？ rather than なりましたの？